1. Field of the Invention
This invention relates to a process for producing a conductive rubber roller used in image forming apparatus such as an electrophotographic copying apparatus, a printer and an electrostatic recording apparatus, and also to a roller for electrophotographic apparatus, such as a transfer roller, set in an image forming apparatus in which a transferable image composed of a toner image is formed and held on an image bearing member such as a photosensitive member by an imaging means such as an electrophotographic process or an electrostatic recording process, and transferred to a transfer medium such as paper.
2. Related Background Art
Conductive rubber rollers such as a charging roller, a transfer roller and a developing roller are used in many of image forming apparatus of an electrophotographic system, such as copying machines and printers. To provide these rollers with conductivity, a method is available in which a conductive filler such as carbon black is added, or in which a conductive rubber material such as acrylonitrile-butadiene rubber or epichlorohydrin rubber is compounded. These roller are each kept in contact with a photosensitive drum under application of a load, and also these rollers are electrified for a long time on account of their use. Accordingly, it is desirable for them to be made of a rubber material small in resistivity variation, and because of problems in production processes, rubber materials such as acrylonitrile-butadiene rubber and epichlorohydrin rubber are widely used in the transfer roller and the charging roller (see, e.g., Japanese Patent Applications Laid-open No. H10-171210 and No. 2002-070835).
The rubber material used for these rollers is kneaded together with a vulcanizing agent, a blowing agent, a filler and so forth so as to be made into a raw-material composition, which is then made into an unvulcanized, cylindrical rubber molded or extruded product by means of a mold, an extruder or the like, and thereafter this molded or extruded product is vulcanized and foamed by heating to make up a cylindrical foam. Thereafter, a core material is press-fitted to the cylindrical foam and then the peripheral surface of the foam is ground to have the shape of a roller. Such a method is used.
As methods for producing these conductive rubber rollers, the following are conventionally available: vulcanization by means of a vulcanizer using a high-pressure steam (see, e.g., Japanese Patent Application Laid-open No. H11-114978), mold vulcanization carried out using a cylindrical mold (see, e.g., Japanese Patent Application Laid-open No. H11-201140), and UHF vulcanization carried out by microwave irradiation (see, e.g., Japanese Patent Application Laid-open No. 2002-221859). In these methods, e.g., in the method of carrying out vulcanization by means of a vulcanizer, the cells in the roller foam obtained are so non-uniform that grinding must be carried out in a large quantity in order to expose the desired cells to the surface. In the molding vulcanization carried out using a cylindrical mold, it takes time to make preparations therefor, besides mold cleaning must be carried out. Hence, it has been unsuitable for producing rollers in a large number.
Firstly, although the method making use of UHF vulcanization can be easily prepared and provides uniform cells, the tube formed may collapse when the rubber is softened, resulting in non-uniform aspect ratios of tube inner and outer diameters. This non-uniformity of the tube has created the non-uniformity of hardness and electrical resistance in the peripheral direction. In order to eliminate this non-uniformity of the tube, a method is known in which short UHF units are connected so as to slope the output of microwave irradiation. However, this method requires a long and large apparatus and takes a long time to excessively irradiate the tube with microwaves, whereby the properties of the rubber materials, epichlorohydrin rubber and acrylonitrile-butadiene rubber, are changed, resulting in the high volume resistivity of the rubber material. Hence, the above method has been unsuitable for the conductive rubber rollers used in copying machines, printers and the like, and also there has been no presentation for any technical development directed to small-diameter rollers whose properties such as resistivity are required to be delicately adjusted. To this vulcanized rubber tube, a conductive core material coated at its preset position with a conductive adhesive is press-fitted, followed by heat treatment, where the rubber tube may partially come off because of non-uniformity of the adhesive, thereby resulting in non-uniform charging. Further, taking into account handling of an organic solvent contained in the adhesive and environmental problems, it is desired to combine the vulcanized rubber tube and the conductive core material together without using any adhesive. It is possible to fasten the vulcanized rubber tube to the conductive core material at a certain pressure, to thereby combine them together without using any adhesive. However, in conventional techniques, the dimensional stability of inner diameter is insufficient so that further improvement in precision has been sought for products. Because of such a background, in the production of conductive rubber rollers used in copying machines, printers and so forth, required to be free from the non-uniformity of hardness and electrical resistance in the peripheral direction, it is sought to provide a production process which is easily prepared for production steps and has good productivity.
Secondly, although the UHF vulcanization can be easily prepared and provides uniform cells, the rubber softens to come to have a larger area in contact with a conveyor and a roller when the rubber tube is heated in a furnace, thereby creating local non-uniformity in foaming. Especially where the rubber softens greatly, the rubber tube deforms to change in its inner diameter, resulting in a poor yield of the rubber tube to bring about an economical problem. Further, the foaming non-uniformity occurring in the rubber tube has been the cause of the non-uniformity of hardness and electrical resistance in the peripheral direction (see Japanese Patent Application Laid-open No. 2002-221859). Meanwhile, it is reported that a rubber tube having a double-layer structure is used and an inner-layer rubber composition is selectively vulcanized to retain the inner diameter of the tube (see Japanese Patent Application Laid-open No. 2003-246485). However, this has not remedied the foaming non-uniformity. In all the cases presented above, no sufficient analysis is made in regard to the foaming non-uniformity, and hence the methods are insufficient for producing a roller having uniform cells.
Thirdly, although the UHF vulcanization can be easily prepared and also provides uniform cells, the rubber softens to come to have a larger area in contact with a conveyor and a roller when the rubber tube is heated in a furnace, thereby creating local non-uniformity in foaming. Especially where the rubber softens greatly, the rubber tube deforms to change in its inner diameter, resulting in a poor yield of the rubber tube to bring about an economical problem. Further, the foaming non-uniformity occurring in the rubber tube has been the cause of the non-uniformity of hardness and electrical resistance in the peripheral direction (see Japanese Patent Application Laid-open No. 2002-221859). In all the cases presented as above, no sufficient analysis is made in regard to the foaming non-uniformity, and hence the methods are insufficient for producing a roller having uniform cells.
Where the above rubber material is heated with microwaves, the heating level for the rubber can be controlled by changing a coefficient of dielectric loss expressed by the product ∈r·tan δ of dielectric constant (∈r) by dielectric power factor (tan δ). Taking note of this coefficient of dielectric loss, a method is reported in which the vulcanization is carried out in the state that conductive carbon black is added and compounded to a rubber component having a small coefficient of dielectric loss (see Japanese Patent Applications Laid-open No. H06-344510 and No. H10-309725). In this case, a technique is employed in which a coefficient of dielectric loss is made as large as 1.0 in order to heat a non-polar natural rubber. However, in a system containing a polar rubber as specified in the present invention, when a coefficient of dielectric loss is increased to be as large as 1.0, the rubber is overheated in some cases.
Fourthly, although the UHF vulcanization can be easily prepared and also provides uniform cells, the rubber softens to come to have a larger area in contact with a conveyor or a roller when the rubber tube is heated in a furnace. Especially in the initial stage of the vulcanization, the viscosity of the rubber decreases so greatly as to bring about such a problem that the rubber tube adheres to the conveyor and roller and, especially in the case of the roller, it winds around the roller. This has caused a lowering of the yield in the vulcanization step and a decrease in the operation rate, to bring about an economical problem. Further, the foaming non-uniformity may occur at the contact surface between the rubber tube and the conveyor or roller. This has been the cause of non-uniform hardness and electrical resistance in the peripheral direction. Also, in the cylindrical grinding for imparting a roller shape, the grinding is carried out in a large quantity in order not to leave non-uniformly ground portions, so that the rubber material must be discarded in a large quantity, bringing about an economical problem and an environmental problem as well (see, e.g., Japanese Patent Application Laid-open No. 2002-221859).
Fifthly, in order to provide these rubber rollers with conductivity, the following methods are conventionally available: a method in which a conductive filler such as carbon black is added and a method in which epichlorohydrin rubber is mixed in acrylonitrile-butadiene rubber to reduce the resistivity in virtue of the epichlorohydrin rubber. However, a problem is raised in that the mixing of epichlorohydrin rubber in a large quantity results in great variations in resistivity due to a change in environment such as temperature and humidity. Further, it is known that, if a mixture with a large quantity of epichlorohydrin rubber mixed therein is irradiated with microwaves, the backbone chain ether linkages are broken and the rubber may come to soften and deteriorate, resulting in unstable hardness of the roller. From such a background, in the production of conductive rubber rollers used in copying machines, printers and so forth, for which it is required that cells in the foamed rubber layer are uniform, there is no non-uniformity of hardness and electrical resistance in the peripheral direction, variations in resistivity in low-resistance ranges because of a change in environment such as temperature and humidity are small and the hardness is stable, it is sought to provide a production process which is easily prepared for production steps and good in productivity.